1. Field of the Invention
This invention relates to imaging of the human iris, as may be used for biometric identification.
2. Description of the Related Art
As traditional forms of personal identification become vulnerable to advancing technology, biometric identification is increasingly seen as a viable approach to personal identification. Techniques such as voice recognition, fingerprinting, and iris imaging rely on physical personal traits that are difficult to change or duplicate.
However, biometric identification via iris imaging typically requires a high resolution image of the iris in order to resolve the fine details necessary to make a positive identification. An image of an iris with approximately 200 micron or better spatial resolution typically is required to uniquely distinguish the fine muscle structure of human irises, as may be required for identification purposes. In systems where the subject is actively cooperating, conditions such as illumination geometry, camera resolution, exposure time, and wavelength of light can be optimized in order to capture a high contrast image of the fine structure of the iris. Existing systems typically require a subject to hold his head in a specific position while staring at the iris imaging camera from close proximity and at a nearly head-on aspect. Although recent advances have been made in iris imaging, the task of capturing sufficiently high resolution images of the human iris generally still requires a fair degree of active cooperation from the subject.
For example, a system using commercial color CCD technology (e.g., 5 megapixels) would typically have a field of view of approximately 15 cm at a 1 m standoff range, yielding a spatial resolution of approximately 75 microns per pixel at the 1 m standoff range. Thus, the subject would have to be within approximately 1 m of the camera and would have to position his iris within the 15 cm field of view for a long enough period of time in order for the camera to focus and capture an adequate resolution image of the iris. This typically requires the subject's active cooperation. The situation becomes significantly worse at longer standoffs. For example, if the same camera were used at a standoff of 10 m, maintaining the same angular resolution would result in a spatial resolution of 750 μm per pixel, which is unacceptable. On the other hand, maintaining a spatial resolution of 75 μm per pixel would result in a 15 cm wide field of view at 10 m. Keeping the iris within this field of view is also very difficult.
The “capture volume” of an iris imaging system is the volume over which the iris imaging system can capture iris images of sufficiently high resolution. The CCD-based system described above and other similar traditional systems have a small capture volume—so small as to make traditional iris imaging systems unsuitable for use in uncooperative situations, such as iris imaging over large groups of people, over longer standoff distances, or for covert identification applications. For example, it may be desirable to capture iris images of subjects as they walk through a portal, such as a metal detector, or in places like airports, train stations, border crossings, secure building entrances and the like. The high-resolution and longer standoff requirements in these applications place significant challenges on iris imaging systems that cannot be met by current designs. The capture volume and standoff capabilities of current iris imaging systems are not large enough to efficiently address these types of situations.
Therefore, there is a need for iris imaging systems that have larger capture volumes and/or can be used at longer standoff distances.